This invention relates generally to inkjet printers and particularly to inkjet printers that can print in a spray-mode.
Two ever-present and normally conflicting goals of inkjet printing is that what is printed should be as life-like or sharp as possible and also that the printing process should be as fast as possible. One way to increase both the sharpness of text and the lifelikeness of non-text images is to increase the print resolution, that is, the number of ink drops that are applied per unit printing area and are accurately placed with separation within that area. The unit printing area is typically the so-called "pixel," which is the common abbreviation for "picture element."
The conventional way of controlling the number of ink dots per pixel is to improve the repeatable accuracy in the "aim" of every printhead in the inkjet printer. One problem with this is that every time an ink drop is fired from the printhead, it takes time for the meniscus of the printing fluid (the "ink") in the printhead to settle down and level out enough to fire the next drop accurately. In such conventional printheads, it is also important for the meniscus to return to a stable initial condition so that the drops that are fired will be of approximately the same size.
In order to damp out and avoid resonant modes on the meniscus and thereby to try to ensure that the ink drops are ejected straight and placed accurately within a pixel, conventional printheads lower the firing velocity and the firing rate based substantially on the viscosity of the ink. Normal firing velocities are on the order of ten meters per second (m/s); and common firing rates are less than 10 kHz.
U.S. Pat. No. 4,503,445 (Tacklind, Mar. 5, 1985) describes a thermal inkjet printer that emits discrete drops whose volume varies to create a printed gray scale. In the Tacklind inkjet printer, ink droplets are fired in groups, so that each subsequent droplet in a group is fired before the previous one separates from the meniscus of the ink in the printhead. The individual droplets within each group then merge on the surface of the medium to be printed because all the droplets are designed to travel along the same direction to go to the same area. Darker pixels can be created by increasing the number of droplets fired into them.
Although the Tacklind system makes it possible to use a heating element for firing the drops that are physically undersized for the particular ink being used, it still relies on an ability to fire droplets straight, that is, in a direction normal to an ejection plane of the printhead, which is typically also normal to the pixel. The maximum single-droplet emission rate for the ink used in the basic embodiment of the Tacklind system is roughly 10 kHz. This typical rate is much lower than the firing rate that one could achieve by the printhead.
In order to increase printing speed even further, what is needed is a way to operate an inkjet printer at a rate much higher than is required for the meniscus of the ink to settle down, which is substantially the maximum possible rates of conventional systems such as Tacklind's.